Apparatus of the above kind is employed, in particular, for enabling separate adjustments to be made in the temperature of the two streams of air which are to be delivered, one into the left hand half and the other into the right hand half of the cabin of the vehicle, according to the respective requirements of the driver on one side of the cabin and a passenger on the other side. In these known forms of apparatus, when the flow of fluid in one of the heat exchangers increases, the amount of fluid available for flow in the other heat exchanger diminishes, and vice versa. As a result, any change in the adjustment relating to one side of the cabin has the secondary effect of also changing the temperature of the air on the other side of the cabin.